Re: [PATCH v5] drm/fbdev-generic: prohibit potential out-of-bounds access

[Sui Jingfeng]

Hi,

On 2023/4/20 15:04, Thomas Zimmermann wrote:
> Hi,
>
> this patch looks to me good and I'd like to merge it, if no one objects.
>
> In the near future, after i915 has switched to struct drm_client, I 
> intend to move DRM's deferred-I/O helpers into fbdev-generic and i915. 


That sound fine,  I can help to test then.

But I think,  it may better to make it a 100% DRM function?


> Those are the two users, but they are fairly different. They can then 
> both have something tailored towards their needs.
>
> Best regards
> Thomas
>
> Am 20.04.23 um 05:05 schrieb Sui Jingfeng:
> > The fbdev test of IGT may write after EOF, which lead to out-of-bound
> > access for drm drivers hire fbdev-generic. For example, run fbdev test
> > on a x86+ast2400 platform, with 1680x1050 resolution, will cause the
> > linux kernel hang with the following call trace:
> >
> >    Oops: 0000 [#1] PREEMPT SMP PTI
> >    [IGT] fbdev: starting subtest eof
> >    Workqueue: events drm_fb_helper_damage_work [drm_kms_helper]
> >    [IGT] fbdev: starting subtest nullptr
> >
> >    RIP: 0010:memcpy_erms+0xa/0x20
> >    RSP: 0018:ffffa17d40167d98 EFLAGS: 00010246
> >    RAX: ffffa17d4eb7fa80 RBX: ffffa17d40e0aa80 RCX: 00000000000014c0
> >    RDX: 0000000000001a40 RSI: ffffa17d40e0b000 RDI: ffffa17d4eb80000
> >    RBP: ffffa17d40167e20 R08: 0000000000000000 R09: ffff89522ecff8c0
> >    R10: ffffa17d4e4c5000 R11: 0000000000000000 R12: ffffa17d4eb7fa80
> >    R13: 0000000000001a40 R14: 000000000000041a R15: ffffa17d40167e30
> >    FS:  0000000000000000(0000) GS:ffff895257380000(0000) 
> > knlGS:0000000000000000
> >    CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >    CR2: ffffa17d40e0b000 CR3: 00000001eaeca006 CR4: 00000000001706e0
> >    Call Trace:
> >     <TASK>
> >     ? drm_fbdev_generic_helper_fb_dirty+0x207/0x330 [drm_kms_helper]
> >     drm_fb_helper_damage_work+0x8f/0x170 [drm_kms_helper]
> >     process_one_work+0x21f/0x430
> >     worker_thread+0x4e/0x3c0
> >     ? __pfx_worker_thread+0x10/0x10
> >     kthread+0xf4/0x120
> >     ? __pfx_kthread+0x10/0x10
> >     ret_from_fork+0x2c/0x50
> >     </TASK>
> >    CR2: ffffa17d40e0b000
> >    ---[ end trace 0000000000000000 ]---
> >
> > The is because damage rectangles computed by
> > drm_fb_helper_memory_range_to_clip() function does not guaranteed to be
> > bound in the screen's active display area. Possible reasons are:
> >
> > 1) Buffers are allocated in the granularity of page size, for mmap 
> > system
> >     call support. The shadow screen buffer consumed by fbdev 
> > emulation may
> >     also choosed be page size aligned.
> >
> > 2) The DIV_ROUND_UP() used in drm_fb_helper_memory_range_to_clip()
> >     will introduce off-by-one error.
> >
> > For example, on a 16KB page size system, in order to store a 1920x1080
> > XRGB framebuffer, we need allocate 507 pages. Unfortunately, the size
> > 1920*1080*4 can not be divided exactly by 16KB.
> >
> >   1920 * 1080 * 4 = 8294400 bytes
> >   506 * 16 * 1024 = 8290304 bytes
> >   507 * 16 * 1024 = 8306688 bytes
> >
> >   line_length = 1920*4 = 7680 bytes
> >
> >   507 * 16 * 1024 / 7680 = 1081.6
> >
> >   off / line_length = 507 * 16 * 1024 / 7680 = 1081
> >   DIV_ROUND_UP(507 * 16 * 1024, 7680) will yeild 1082
> >
> > memcpy_toio() typically issue the copy line by line, when copy the last
> > line, out-of-bound access will be happen. Because:
> >
> >   1082 * line_length = 1082 * 7680 = 8309760, and 8309760 > 8306688
> >
> > Note that userspace may stil write to the invisiable area if a larger
> > buffer than width x stride is exposed. But it is not a big issue as
> > long as there still have memory resolve the access if not drafting so
> > far.
> >
> >   - Also limit the y1 (Daniel)
> >   - keep fix patch it to minimal (Daniel)
> >   - screen_size is page size aligned because of it need mmap (Thomas)
> >   - Adding fixes tag (Thomas)
> >
> > Fixes: aa15c677cc34 ("drm/fb-helper: Fix vertical damage clipping")
> >
> > Signed-off-by: Sui Jingfeng <suijingfeng@xxxxxxxxxxx>
> > Reviewed-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
> > Tested-by: Geert Uytterhoeven <geert+renesas@xxxxxxxxx>
> > Link: 
> > https://lore.kernel.org/dri-devel/ad44df29-3241-0d9e-e708-b0338bf3c623@xxxxxx/
> > ---
> >   drivers/gpu/drm/drm_fb_helper.c | 16 ++++++++++++----
> >   1 file changed, 12 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_fb_helper.c 
> > b/drivers/gpu/drm/drm_fb_helper.c
> > index 64458982be40..6bb1b8b27d7a 100644
> > --- a/drivers/gpu/drm/drm_fb_helper.c
> > +++ b/drivers/gpu/drm/drm_fb_helper.c
> > @@ -641,19 +641,27 @@ static void drm_fb_helper_damage(struct 
> > drm_fb_helper *helper, u32 x, u32 y,
> >   static void drm_fb_helper_memory_range_to_clip(struct fb_info 
> > *info, off_t off, size_t len,
> >                              struct drm_rect *clip)
> >   {
> > +    u32 line_length = info->fix.line_length;
> > +    u32 fb_height = info->var.yres;
> >       off_t end = off + len;
> >       u32 x1 = 0;
> > -    u32 y1 = off / info->fix.line_length;
> > +    u32 y1 = off / line_length;
> >       u32 x2 = info->var.xres;
> > -    u32 y2 = DIV_ROUND_UP(end, info->fix.line_length);
> > +    u32 y2 = DIV_ROUND_UP(end, line_length);
> > +
> > +    /* Don't allow any of them beyond the bottom bound of display 
> > area */
> > +    if (y1 > fb_height)
> > +        y1 = fb_height;
> > +    if (y2 > fb_height)
> > +        y2 = fb_height;
> >         if ((y2 - y1) == 1) {
> >           /*
> >            * We've only written to a single scanline. Try to reduce
> >            * the number of horizontal pixels that need an update.
> >            */
> > -        off_t bit_off = (off % info->fix.line_length) * 8;
> > -        off_t bit_end = (end % info->fix.line_length) * 8;
> > +        off_t bit_off = (off % line_length) * 8;
> > +        off_t bit_end = (end % line_length) * 8;
> >             x1 = bit_off / info->var.bits_per_pixel;
> >           x2 = DIV_ROUND_UP(bit_end, info->var.bits_per_pixel);